A fuel cell system is an energy conversion system which supplies a fuel gas and an oxidizing gas to a membrane electrode assembly to cause an electrochemical reaction, thereby converting chemical energy into electric energy. Above all, a solid polymer electrolyte type fuel cell using a solid polymer membrane as an electrolyte can easily be made compact at a low cast, and additionally the fuel cell has a high output density, whereby a use application as a car-mounted power source system is expected.
Output characteristics (current-voltage characteristics) of the fuel cell are not constant, and constantly fluctuate in accordance with a stack temperature, and the flow rate, pressure and humidity of a reactant gas and further in accordance with the fluctuation of an internal resistance due to the variance of water in a polymer electrolyte membrane and the like. Moreover, it is known that the output characteristics fluctuate depending on whether the operation state of the fuel cell is an excessive state or a steady state and that the output characteristics also noticeably fluctuate in accordance with aged deterioration or the like. In such situations, the constantly fluctuating output characteristics need to be grasped so as to appropriately control the operation of the fuel cell. In Japanese Patent Application Laid-Open No. 2004-241272, there is disclosed a method for estimating the output characteristics of the fuel cell based on the output current and output voltage of the fuel cell, to control a cell operation.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-241272